Handling `break` and `continue` in trailing macro body

[cbuttner]

Just wanted to bring this issue I've had up for discussion.
When using trailing macro bodies to implement iteration, break and continue don't apply to the macro body and can't be used the same as in a regular for-loop.

Example:

macro @foreach(int[] list; @body(int)) {
  foreach (v : list) {
    @body(v);
  }
}

fn void test() {
  @foreach(int[] {1,2,3}; int v) {
    break; // Error: There is no valid target for 'break', did you make a mistake?
  };
}

Is there some way we can "handle" break and continue in a better way to make iteration macros more useful?

I understand there's problems with merely including break in the @body. Sometimes you have nested loops in the macro and break should break the outer loop. Or, there may not even be a for loop in the macro.

[lerno]

The problem here is that whether or not a "break" (or "continue") exists is entirely opaque to the user of the function. At one point in time I was thinking of a way to kind of attach it somehow to the body as an attribute to allow break to work. However, that seemed too much work to make it work.

There are two workarounds:

1. Create an iterator that encapsulates the macro behaviour. This is how the iterator on HashMap works.
2. Use {| |} for breaks:

fn void test() 
{
   {|
      @foreach(int[] {1,2,3}; int v) 
      {
         return;
      };
   |}
}